Constructing a magnet with multiple north-south poles is well-known in the art. For example, U.S. Pat. No. 3,127,544 issued to Blume, Jr., discloses an apparatus for magnetizing permanent magnet materials to form band-like poles. Blume discloses an apparatus comprised of an upper and a lower assembly. Each assembly has a north primary pole piece, a south primary pole piece and an electro-magnetic coil which establishes a magnetic potential difference between the primary pole pieces. These two assemblies, as their electro-magnets are activated, form alternate north-south magnetic poles on magnetic material passed between the assemblies. However, using this configuration, the north-south poles are not well-defined. This lack of definition is caused by a phenomena referred to as a "Bloch wall". When a "Bloch wall" occurs, the transition from one polarity to another is accompanied by a decrease in the magnetic field and a gradual switching from north to south and from south to north poles. This transition requires a finite distance through the material in which to occur so as to complete the switch. Therefore, the distance associated with the polarity switch fails to have well-defined poles.
Another prior art multi-pole magnet, U.S. Pat. No. 4,513,216 issued to Mueller, teaches a multi-pole rotor having its multiple north-south fields on its circumference. Mueller discloses a rotor having a minimum of three pieces, two of which are crown gears and one of which is a ferrous ring which completes a magnetic circuit on the internal side. Depending upon the cylindrical height of the particular rotor, Mueller uses spacer material, referred to as pole carriers, for structural strength. Hence, Mueller's assembly consists of many parts which present considerable problems in manufacturing, particularly significantly increasing material and labor costs.
Therefore, a need arises for a magnet which is easily magnetized, having a minimum number of pieces and exhibiting multi-poles which are well-defined.
Many types of encoders, including optical, mechanical, and magnetic, are also well-known in the art. Optical encoders provide high resolution, but have the disadvantages of high cost and the requirement of a clean working environment. Mechanical encoders are generally low cost items, but have only fair resolution and must also be operated in a clean environment for optimal performance. In contrast, magnetic encoders are not only constructed out of low cost material, but also perform under hostile environmental conditions.
However, present magnetic encoders, specifically dynamic magnetic encoders, are constructed with ferrous gear teeth or any high permeable metal (such as steel) protrusions. (Dynamic encoders require movement to decode, i.e. when motion ceases so does the output signal.) The size, i.e. thickness, of the teeth or protrusions of the dynamic, incremental magnetic encoder severely limits the resolution of the system. These same teeth or protrusions create numerous manufacturing difficulties including mechanical alignment problems and increased assembly cost. Other magnetic encoders currently used, for example static magnetic encoders which may transmit information with or without movement, have magnets constructed from a solid piece, thereby suffering from the same problem of pole definition described above for Blume.
Furthermore, as noted above, both of the above presently used magnetic encoders are incremental encoders, i.e. provide only a relative count with respect to an index point, which considerably limits their use.
Therefore, a need arises for an encoder which is low cost, operates in a hostile environment, and has a medium resolution. A further need arises for an absolute encoder which, unlike an incremental encoder, provides an accurate readout regardless of position, i.e. no index position is required.